The use of Radio Frequency Identification (RFID) tags are quickly gaining popularity for use in the monitoring and tracking of an item. RFID technology allows a user to remotely store and retrieve data in connection with an item utilizing a small, unobtrusive tag. As an RFID tag operates in the radio frequency (RF) portion of the electromagnetic spectrum, an electromagnetic or electrostatic coupling can occur between an RFID tag affixed to an item and an RFID tag reader. This coupling is advantageous, as it precludes the need for a direct contact or line of sight connection between the tag and the reader.
In some currently used passive and semi-passive RFID tags, during the ‘read’ cycle, the reader generally transmits a continuous unmodulated carrier signal. A distant RFID tag includes a RF switch connected to the tag's antenna, which repetitively alternates its state at a rate called the ‘backscatter link frequency’ (BLF). This RF switch effectively modulates the carrier signal in the tag received from the transmitter, creating sidebands surrounding the carrier frequency, and separated from the carrier frequency by the backscatter link frequency. For example, if the carrier frequency is 900 MHz and the tag backscatter modulation is at 160 KHz, side bands present in the return signal are about 900 MHz+˜160 KHz and 900 MHz−˜160 KHz. These sidebands are re-radiated by the tag's antenna, and are recovered by the reader, e.g., the reader detects and demodulates one or both of the side bands to obtain the data returned by the tag.
The above description is one typical way in which the tag communicates information to the reader. The tag does not create RF power, but instead modulates incoming RF power from the reader's transmitter, and in so doing, converts some of that incoming power to sideband frequencies which can be separately recovered by the reader. These backscatter sidebands only exist when (and because) the reader is transmitting.
The current RFID standard includes a wide range of reverse link parameters such as backscatter link frequency and data rates to provide flexibility in various applications. Most readers are designed to use specific parameter combinations at which the demodulator operates, i.e., the readers are designed to operate at a specific frequency and data rate. Such designs have traditionally been favored due to their simplicity, lower cost, and effectiveness at talking to particular tag designs. Moreover, in long distance communications with semi-passive and active tags, the low return signal strength urges towards settings directed to a specific frequency. However, such implementations are unable to cover a continuum of reverse link backscatter link frequencies and data rates.